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Cite this: Chem. Commun., 2014, 50, 14889
Fabrication of porous Co3O4 nanowires with high CO sensing performance at a low operating temperature†
Received 16th July 2014, Accepted 2nd October 2014
Zhifeng Dou,a Changyan Cao,b Yong Chena and Weiguo Song*b
DOI: 10.1039/c4cc05498a www.rsc.org/chemcomm
The porous Co3O4 nanowires were fabricated using a fluoride anionassisted hydrothermal and controlled annealing route. The nanowires showed superior CO gas-sensing performances, such as high selectivity against CO and optimal sensing activity at a relatively low operating temperature (Top r 100 8C). Such properties were ascribed to fluoride doping and porous nanowire structure.
Gas sensors were widely used in the fields of environmental protection, emission monitoring, public safety and human health to detect and monitor various gases and vapours, including explosives or toxic gases, and odours.1–5 Metal oxide semiconductor-based gas sensors, as some of the most important gas sensors, have been extensively investigated due to their advantages of low cost, high stability, extensive application, and ease of integration.6–11 However, the metal oxide semiconductor-based sensing materials usually require higher operating temperatures (usually well above 200 1C) to achieve their optimal sensing performances.12 The higher operating temperature reduces response selectivity, shortens sensor lifetime and increases the cost of practical applications. Higher operating temperature metal oxide semiconductorbased sensors may also become a potentially dangerous source when used as monitoring devices for flammable and explosive gas. In addition, a higher operating temperature can deteriorate reproducibility of gas sensors due to sintering-induced grain growth of nanostructured gas-sensing materials.13 Hence, it is very useful to develop metal oxide semiconductor-based sensing materials with a low operating temperature or even room temperature and high gas-sensing performance. Currently, a
Key Laboratory of Ministry of Education for Advanced Materials in Tropical Island Resources, College of Materials and Chemical Engineering, Hainan University, Haikou, 570228, P. R. China. E-mail: [email protected]; Fax: +86-0898-66168037; Tel: +86-0898-66168037 b Beijing National Laboratory for Molecular Sciences (BNLMS) and Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China. E-mail: [email protected]; Fax: +86-010-62557908; Tel: +86-010-62557908 † Electronic supplementary information (ESI) available: SEM images, XRD patterns, Raman and XPS spectra, nitrogen adsorption and desorption isotherms, TG and DTG curves, as well as experimental details. See DOI: 10.1039/c4cc05498a
This journal is © The Royal Society of Chemistry 2014
some effective strategies, such as noble metal sensitization,14–19 nanocomposites,12,20–23 non-noble metal doping24,25 and so on, have been reported to reduce the operating temperature of the metal oxide semiconductor sensing materials. Tricobalt tetraoxide (Co3O4) is an important p-type semiconductor transition metal oxide (Eg E 1.48–2.19 eV). It possesses excellent catalytic properties and electrical properties for gas sensing.26–30 However, like other metal oxide gas sensitive materials, Co3O4 shows low response sensitivity and poor response selectivity at a low operating temperature. In this work, we developed a hydrothermal and controlled annealing route to produce porous Co3O4 nanowires that exhibited excellent CO sensing performance at a low operating temperature (Top r 100 1C). A simple fluoride anion-assisted hydrothermal method was used to obtain a light pink flocculent precipitate, and then the above precipitate was converted to the gray black final product by thermal annealing in air (for the detailed fabricating method, see the ESI†). The crystallographic structure and phase purity of the as-prepared light pink flocculent precipitate and gray black final products were examined by X-ray powder diffraction (XRD). The XRD patterns (Fig. 1) indicated that intermediate and final products were pure cobalt carbonate hydroxide hydrate (Co(CO3)0.5(OH)0.11H2O, orthorhombic, JCPDS 48-0083) and tricobalt tetraoxide (Co3O4, cubic, JCPDS 43-1003), respectively. The morphologies of the products were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Fig. 2a, the hydrothermal product was uniform nanowires with about 100 nm in diameter. Fig. 2b showed that the nanowire morphology remained after annealing in air, but the surfaces of the nanowires were rough with a large number of cracks. The results of SAED and TEM images (Fig. 2c) revealed that a single Co3O4 nanowire had a quasi-monocrystalline structure with abundant pores. As shown in Fig. 2d–h, a series of Co3O4 nanowires were fabricated by annealing as-prepared cobalt carbonate hydroxide hydrate nanowires at 300 1C, 350 1C, 400 1C, 450 1C and 500 1C, respectively (corresponding products denoted S300, S350, S400, S450 and S500). TEM images showed that cracked particles
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Fig. 2 (a) SEM image of the hydrothermal products; (b) SEM image of the final products; (c) TEM and SAED images of a single nanowire obtained at 350 1C; (d)–(h) TEM images of the porous nanowires prepared at different annealing temperatures.
morphologies of as-obtained Co3O4 nanowires were sensitive to the annealing temperature and their microstructures could be regulated by controlling the annealing conditions. The thermal decomposition behaviour of the precursors was examined by thermogravimetric analysis (TGA). The results showed that there were three main weight loss stages (Fig. S7, ESI†). The first weight loss of about 2% before 250 1C was mainly due to the elimination of absorbed water and crystallization water of the sample. Subsequently, there was a distinct weight loss of about 21% between 250 1C and 360 1C. The weight loss was ascribed to the release of CO2 and H2O during solid thermal decomposition of the precursors. In this stage, the abundant fractured porous structures were formed accompanying gas evolution and solid volume contraction. As the temperature was increased to 360 1C, fractured porous structures decreased gradually with primary small nanoparticles grown up into larger particles. The sensing performances of the as-prepared porous Co3O4 nanowires were investigated. Firstly, the above Co3O4 nanowire samples fabricated at different annealing temperatures were tested for optimum operating temperature sensing CO. As shown in Fig. 3a, the optimum operating temperature of every sample except S500 was approximately 100 1C, which was much lower than that of other Co3O4 gas sensing materials in the literature (Table S1, ESI†). All samples, especially S300 and S350, also exhibited superior gas response sensitivity (defined as the ratio of the resistance in the test gas (Rgas) to its stationary electrical resistance of the sensor in air (Rair)) at a lower operating temperature (Top o 100 1C). At the same time, all samples had much higher response sensitivity than commercial Co3O4 (denoted C-Co3O4) in the range of 50–200 1C. The relation curves between response sensitivity and CO gas concentration of the as-prepared nanowires were then tested at 100 1C (Fig. 3b). The results showed that the sensitivities of all five as-prepared nanowires, especially S350 and S400, were much higher than that of C-Co3O4 in the range of 10 to 200 ppm CO. The response–recovery times of all samples were calculated under 50 ppm CO at 100 1C. Fig. S8a and b (ESI†) revealed that the response–recovery times of as-prepared nanowires shortened gradually with an increasing annealing temperature and were all shorter than that of C-Co3O4. Overall, S350 exhibited the optimal CO gas-sensing. The response stability of S350 was tested at 100 1C operating temperature (Fig. 4a). The results showed that there was almost no apparent signal attenuation after ten on–off cycles. The gas-sensing performances
gradually grew up (Fig. 2d–h), but the diameters became smaller as annealing temperature increased from 300 1C to 500 1C (Fig. S1, ESI†). XRD patterns and Raman spectra showed that S300–S500 were pure tricobalt tetraoxide phases with a little difference in crystallinity (Fig. S2 and S3, ESI†). Nitrogen adsorption and desorption isotherms of S300–S500 displayed a typical type IV adsorption isotherm with a H3-type hysteresis loop at different relative pressure ranges (Fig. S4, ESI†). The specific surface areas (SBET) gradually reduced as annealing temperature elevated (Fig. S5, ESI†). In addition, the mesopore volume of the samples also decreased as annealing temperature elevated (Fig. S6, ESI†). These results indicated that the
Fig. 3 (a) Gas response sensitivities versus operating temperature at 50 ppm CO concentration; (b) real-time CO gas sensing characterization based on C-Co3O4 and the different as-prepared Co3O4 nanowires.
Fig. 1 XRD patterns of the light pink hydrothermal products (a) and the products calcined in air (b).
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against CO. Fluoride doping was found to be the key factor in such enhanced sensing properties. We greatly appreciate the financial support from National Natural Science Foundation of China (NSFC21121063, NSFC51362009) and the Chinese Academy of Sciences.
Published on 02 October 2014. Downloaded by Georgia Institute of Technology on 31/10/2014 09:07:45.
Notes and references Fig. 4 (a) Response stability of S350 at 100 1C; (b) real-time hydrogen sensing characterization based on S350 at 100 1C.
of S350 to hydrogen and methane were also tested under the same experimental conditions (Fig. 4b) in order to characterize gas-sensing selectivity for CO gas. The results showed that the sample had much lower response sensitivity to hydrogen than carbon monoxide and no response to methane under the same experimental conditions. This indicated that as-prepared nanowires had excellent gas-sensing selectivity to CO at a low operating temperature. The above gas-sensing characterization results revealed that the as-prepared porous Co3O4 nanowires possessed excellent CO gas-sensing performances at a low operating temperature. Based on structure–property correlations, excellent gas-sensing might be related to the specific surface chemical composition and microstructure. In general, the porous structure is beneficial to gas sensing performance by increasing the surface reactive sites and facilitating the diffusion of target gases.31–33 Another factor is the close relationship between chemical composition and reducing the operating temperature.23 The surface chemical composition of as-prepared porous Co3O4 nanowires was investigated by X-ray photoelectron spectroscopy (XPS). XPS results (Fig. S9, ESI†) revealed a weak F 1s characteristic peak (684 eV), in addition to main characteristic peaks of Co 2p1/2 (796 eV), Co 2p3/2 (780 eV) and O 1s (530 eV). The fluoride species, with its stronger electron binding capacity than oxygen and very similar anion radius between the fluoride anion (F , 133 pm) and oxygen anion (O2 , 140 pm), existed as substitution doping in the p-type Co3O4 semiconductor.34 These fluoride species helped to improve the conductivity of the semiconductor, which was essential to enhance gas-sensing properties of semiconductor gas-sensing materials at a low temperature. In order to verify the effect of surface fluorine doping, control experiments by modifying C-Co3O4 with fluorine doping were conducted (see Experimental section, ESI†). The results demonstrated that fluoride-doped Co3O4 (denoted F-Co3O4) had higher response sensitivity than C-Co3O4 under the same test conditions (Fig. S10, ESI†). The optimum operating temperature of F-Co3O4 tended to be lower than C-Co3O4 (Fig. S11, ESI†). This indicated that the CO gassensing performance of the p-type Co3O4 semiconductor could be greatly improved by the surface fluorine doping method. In summary, we produced fluoride-doped porous Co3O4 nanowires from a fluoride anion assisted hydrothermal method and a subsequent controlled annealing route. In gas-sensing applications, the nanowires showed excellent CO gas-sensing performances compared to other Co3O4 nanostructured materials reported in the literature, including optimal sensitivity at a much lower operating temperature (Top r 100 1C) and excellent selectivity
This journal is © The Royal Society of Chemistry 2014
1 J. Park, X. Shen and G. Wang, Sens. Actuators, B, 2009, 136, 494–498. 2 J. Polleux, A. Gurlo, N. Barsan, U. Weimar, M. Antonietti and M. Niederberger, Angew. Chem., Int. Ed., 2006, 118, 267–271. 3 X. Shen, G. Wang and D. Wexler, Sens. Actuators, B, 2009, 143, 325–332. 4 M. E. Franke, T. J. Koplin and U. Simon, Small, 2006, 2, 36–50. 5 D. Mao, J. Yao, X. Lai, M. Yang, J. Du and D. Wang, Small, 2011, 7, 578–582. 6 A. M. Ruiz, G. Sakai, A. Cornet, K. Shimanoe, J. R. Morante and N. Yamazoe, Sens. Actuators, B, 2003, 93, 509–518. 7 K. X. Yao, R. Sinclair and H. C. Zeng, J. Phys. Chem. C, 2007, 111, 2032–2039. 8 Z. Jiang, J. Wang, L. Meng, Y. Huang and L. Liu, Chem. Commun., 2011, 47, 6350–6352. 9 J.-H. Lee, Sens. Actuators, B, 2009, 140, 319–336. 10 Z. F. Dou, C. Y. Cao, Q. Wang, J. Qu, Y. Yu and W. G. Song, ACS Appl. Mater. Interfaces, 2012, 4, 5698–5703. 11 X. Han, M. Jin, S. Xie, Q. Kuang, Z. Jiang, Y. Jiang, Z. Xie and L. Zheng, Angew. Chem., Int. Ed., 2009, 48, 9180–9183. 12 P. A. Russo, N. Donato, S. G. Leonardi, S. Baek, D. E. Conte, G. Neri and N. Pinna, Angew. Chem., Int. Ed., 2012, 51, 11053–11057. 13 R. L. Vander Wal, G. W. Hunter, J. C. Xu, M. J. Kulis, G. M. Berger and T. M. Ticich, Sens. Actuators, B, 2009, 138, 113–119. 14 S. H. Jeong, S. Kim, J. Cha, M. S. Son, S. H. Park, H.-Y. Kim, M. H. Cho, M.-H. Whangbo, K.-H. Yoo and S.-J. Kim, Nano Lett., 2013, 13, 5938–5943. 15 R. K. Joshi, Q. Hu, F. Alvi, N. Joshi and A. Kumar, J. Phys. Chem. C, 2009, 113, 16199–16202. 16 N. S. Ramgir, P. K. Sharma, N. Datta, M. Kaur, A. K. Debnath, D. K. Aswal and S. K. Gupta, Sens. Actuators, B, 2013, 186, 718–726. 17 P. Manjula, S. Arunkumar and S. V. Manorama, Sens. Actuators, B, 2011, 152, 168–175. 18 K. Wang, T. Zhao, G. Lian, Q. Yu, C. Luan, Q. Wang and D. Cui, Sens. Actuators, B, 2013, 184, 33–39. 19 Z. Wang, Z. Li, T. Jiang, X. Xu and C. Wang, ACS Appl. Mater. Interfaces, 2013, 5, 2013–2021. 20 G. Lu, J. Xu, J. Sun, Y. Yu, Y. Zhang and F. Liu, Sens. Actuators, B, 2012, 162, 82–88. 21 M. Li, M. Hu, D. Jia, S. Ma and W. Yan, Sens. Actuators, B, 2013, 186, 140–147. 22 N. Chen, X. Li, X. Wang, J. Yu, J. Wang, Z. Tang and S. A. Akbar, Sens. Actuators, B, 2013, 188, 902–908. 23 K. Anand, O. Singh, M. P. Singh, J. Kaur and R. C. Singh, Sens. Actuators, B, 2014, 195, 409–415. 24 J. Su, X.-X. Zou, Y.-C. Zou, G.-D. Li, P.-P. Wang and J.-S. Chen, Inorg. Chem., 2013, 52, 5924–5930. 25 L. Han, D. Wang, Y. Lu, T. Jiang, B. Liu and Y. Lin, J. Phys. Chem. C, 2011, 115, 22939–22944. 26 J. Wang, N. Yang, H. Tang, Z. Dong, Q. Jin, M. Yang, D. Kisailus, H. Zhao, Z. Tang and D. Wang, Angew. Chem., Int. Ed., 2013, 52, 6417–6420. 27 C. C. Li, X. M. Yin, T. H. Wang and H. C. Zeng, Chem. Mater., 2009, 21, 4984–4992. 28 W. Y. Li, L. N. Xu and J. Chen, Adv. Funct. Mater., 2005, 15, 851–857. 29 C. Sun, S. Rajasekhara, Y. Chen and J. B. Goodenough, Chem. Commun., 2011, 47, 12852–12854. 30 Q. Jiao, M. Fu, C. You, Y. Zhao and H. Li, Inorg. Chem., 2012, 51, 11513–11520. ¨, W. Zhan, Y. He, Y. Wang, X. Kong, Q. Kuang, Z. Xie and 31 Y. Lu L. Zheng, ACS Appl. Mater. Interfaces, 2014, 6, 4186–4195. 32 Z. Li, X. Lai, H. Wang, D. Mao, C. Xing and D. Wang, J. Phys. Chem. C, 2009, 113, 2792–2797. 33 X. Lai, D. Wang, N. Han, J. Du, J. Li, C. Xing, Y. Chen and X. Li, Chem. Mater., 2010, 22, 3033–3042. 34 A. V. Moholkar, S. M. Pawar, K. Y. Rajpure, C. H. Bhosale and J. H. Kim, Appl. Surf. Sci., 2009, 255, 9358–9364.
Chem. Commun., 2014, 50, 14889--14891 | 14891
Published on 02 October 2014. Downloaded by Georgia Institute of Technology on 31/10/2014 09:07:45.
COMMUNICATION
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Cite this: Chem. Commun., 2014, 50, 14889
Fabrication of porous Co3O4 nanowires with high CO sensing performance at a low operating temperature†
Received 16th July 2014, Accepted 2nd October 2014
Zhifeng Dou,a Changyan Cao,b Yong Chena and Weiguo Song*b
DOI: 10.1039/c4cc05498a www.rsc.org/chemcomm
The porous Co3O4 nanowires were fabricated using a fluoride anionassisted hydrothermal and controlled annealing route. The nanowires showed superior CO gas-sensing performances, such as high selectivity against CO and optimal sensing activity at a relatively low operating temperature (Top r 100 8C). Such properties were ascribed to fluoride doping and porous nanowire structure.
Gas sensors were widely used in the fields of environmental protection, emission monitoring, public safety and human health to detect and monitor various gases and vapours, including explosives or toxic gases, and odours.1–5 Metal oxide semiconductor-based gas sensors, as some of the most important gas sensors, have been extensively investigated due to their advantages of low cost, high stability, extensive application, and ease of integration.6–11 However, the metal oxide semiconductor-based sensing materials usually require higher operating temperatures (usually well above 200 1C) to achieve their optimal sensing performances.12 The higher operating temperature reduces response selectivity, shortens sensor lifetime and increases the cost of practical applications. Higher operating temperature metal oxide semiconductorbased sensors may also become a potentially dangerous source when used as monitoring devices for flammable and explosive gas. In addition, a higher operating temperature can deteriorate reproducibility of gas sensors due to sintering-induced grain growth of nanostructured gas-sensing materials.13 Hence, it is very useful to develop metal oxide semiconductor-based sensing materials with a low operating temperature or even room temperature and high gas-sensing performance. Currently, a
Key Laboratory of Ministry of Education for Advanced Materials in Tropical Island Resources, College of Materials and Chemical Engineering, Hainan University, Haikou, 570228, P. R. China. E-mail: [email protected]; Fax: +86-0898-66168037; Tel: +86-0898-66168037 b Beijing National Laboratory for Molecular Sciences (BNLMS) and Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China. E-mail: [email protected]; Fax: +86-010-62557908; Tel: +86-010-62557908 † Electronic supplementary information (ESI) available: SEM images, XRD patterns, Raman and XPS spectra, nitrogen adsorption and desorption isotherms, TG and DTG curves, as well as experimental details. See DOI: 10.1039/c4cc05498a
This journal is © The Royal Society of Chemistry 2014
some effective strategies, such as noble metal sensitization,14–19 nanocomposites,12,20–23 non-noble metal doping24,25 and so on, have been reported to reduce the operating temperature of the metal oxide semiconductor sensing materials. Tricobalt tetraoxide (Co3O4) is an important p-type semiconductor transition metal oxide (Eg E 1.48–2.19 eV). It possesses excellent catalytic properties and electrical properties for gas sensing.26–30 However, like other metal oxide gas sensitive materials, Co3O4 shows low response sensitivity and poor response selectivity at a low operating temperature. In this work, we developed a hydrothermal and controlled annealing route to produce porous Co3O4 nanowires that exhibited excellent CO sensing performance at a low operating temperature (Top r 100 1C). A simple fluoride anion-assisted hydrothermal method was used to obtain a light pink flocculent precipitate, and then the above precipitate was converted to the gray black final product by thermal annealing in air (for the detailed fabricating method, see the ESI†). The crystallographic structure and phase purity of the as-prepared light pink flocculent precipitate and gray black final products were examined by X-ray powder diffraction (XRD). The XRD patterns (Fig. 1) indicated that intermediate and final products were pure cobalt carbonate hydroxide hydrate (Co(CO3)0.5(OH)0.11H2O, orthorhombic, JCPDS 48-0083) and tricobalt tetraoxide (Co3O4, cubic, JCPDS 43-1003), respectively. The morphologies of the products were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Fig. 2a, the hydrothermal product was uniform nanowires with about 100 nm in diameter. Fig. 2b showed that the nanowire morphology remained after annealing in air, but the surfaces of the nanowires were rough with a large number of cracks. The results of SAED and TEM images (Fig. 2c) revealed that a single Co3O4 nanowire had a quasi-monocrystalline structure with abundant pores. As shown in Fig. 2d–h, a series of Co3O4 nanowires were fabricated by annealing as-prepared cobalt carbonate hydroxide hydrate nanowires at 300 1C, 350 1C, 400 1C, 450 1C and 500 1C, respectively (corresponding products denoted S300, S350, S400, S450 and S500). TEM images showed that cracked particles
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Fig. 2 (a) SEM image of the hydrothermal products; (b) SEM image of the final products; (c) TEM and SAED images of a single nanowire obtained at 350 1C; (d)–(h) TEM images of the porous nanowires prepared at different annealing temperatures.
morphologies of as-obtained Co3O4 nanowires were sensitive to the annealing temperature and their microstructures could be regulated by controlling the annealing conditions. The thermal decomposition behaviour of the precursors was examined by thermogravimetric analysis (TGA). The results showed that there were three main weight loss stages (Fig. S7, ESI†). The first weight loss of about 2% before 250 1C was mainly due to the elimination of absorbed water and crystallization water of the sample. Subsequently, there was a distinct weight loss of about 21% between 250 1C and 360 1C. The weight loss was ascribed to the release of CO2 and H2O during solid thermal decomposition of the precursors. In this stage, the abundant fractured porous structures were formed accompanying gas evolution and solid volume contraction. As the temperature was increased to 360 1C, fractured porous structures decreased gradually with primary small nanoparticles grown up into larger particles. The sensing performances of the as-prepared porous Co3O4 nanowires were investigated. Firstly, the above Co3O4 nanowire samples fabricated at different annealing temperatures were tested for optimum operating temperature sensing CO. As shown in Fig. 3a, the optimum operating temperature of every sample except S500 was approximately 100 1C, which was much lower than that of other Co3O4 gas sensing materials in the literature (Table S1, ESI†). All samples, especially S300 and S350, also exhibited superior gas response sensitivity (defined as the ratio of the resistance in the test gas (Rgas) to its stationary electrical resistance of the sensor in air (Rair)) at a lower operating temperature (Top o 100 1C). At the same time, all samples had much higher response sensitivity than commercial Co3O4 (denoted C-Co3O4) in the range of 50–200 1C. The relation curves between response sensitivity and CO gas concentration of the as-prepared nanowires were then tested at 100 1C (Fig. 3b). The results showed that the sensitivities of all five as-prepared nanowires, especially S350 and S400, were much higher than that of C-Co3O4 in the range of 10 to 200 ppm CO. The response–recovery times of all samples were calculated under 50 ppm CO at 100 1C. Fig. S8a and b (ESI†) revealed that the response–recovery times of as-prepared nanowires shortened gradually with an increasing annealing temperature and were all shorter than that of C-Co3O4. Overall, S350 exhibited the optimal CO gas-sensing. The response stability of S350 was tested at 100 1C operating temperature (Fig. 4a). The results showed that there was almost no apparent signal attenuation after ten on–off cycles. The gas-sensing performances
gradually grew up (Fig. 2d–h), but the diameters became smaller as annealing temperature increased from 300 1C to 500 1C (Fig. S1, ESI†). XRD patterns and Raman spectra showed that S300–S500 were pure tricobalt tetraoxide phases with a little difference in crystallinity (Fig. S2 and S3, ESI†). Nitrogen adsorption and desorption isotherms of S300–S500 displayed a typical type IV adsorption isotherm with a H3-type hysteresis loop at different relative pressure ranges (Fig. S4, ESI†). The specific surface areas (SBET) gradually reduced as annealing temperature elevated (Fig. S5, ESI†). In addition, the mesopore volume of the samples also decreased as annealing temperature elevated (Fig. S6, ESI†). These results indicated that the
Fig. 3 (a) Gas response sensitivities versus operating temperature at 50 ppm CO concentration; (b) real-time CO gas sensing characterization based on C-Co3O4 and the different as-prepared Co3O4 nanowires.
Fig. 1 XRD patterns of the light pink hydrothermal products (a) and the products calcined in air (b).
14890 | Chem. Commun., 2014, 50, 14889--14891
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against CO. Fluoride doping was found to be the key factor in such enhanced sensing properties. We greatly appreciate the financial support from National Natural Science Foundation of China (NSFC21121063, NSFC51362009) and the Chinese Academy of Sciences.
Published on 02 October 2014. Downloaded by Georgia Institute of Technology on 31/10/2014 09:07:45.
Notes and references Fig. 4 (a) Response stability of S350 at 100 1C; (b) real-time hydrogen sensing characterization based on S350 at 100 1C.
of S350 to hydrogen and methane were also tested under the same experimental conditions (Fig. 4b) in order to characterize gas-sensing selectivity for CO gas. The results showed that the sample had much lower response sensitivity to hydrogen than carbon monoxide and no response to methane under the same experimental conditions. This indicated that as-prepared nanowires had excellent gas-sensing selectivity to CO at a low operating temperature. The above gas-sensing characterization results revealed that the as-prepared porous Co3O4 nanowires possessed excellent CO gas-sensing performances at a low operating temperature. Based on structure–property correlations, excellent gas-sensing might be related to the specific surface chemical composition and microstructure. In general, the porous structure is beneficial to gas sensing performance by increasing the surface reactive sites and facilitating the diffusion of target gases.31–33 Another factor is the close relationship between chemical composition and reducing the operating temperature.23 The surface chemical composition of as-prepared porous Co3O4 nanowires was investigated by X-ray photoelectron spectroscopy (XPS). XPS results (Fig. S9, ESI†) revealed a weak F 1s characteristic peak (684 eV), in addition to main characteristic peaks of Co 2p1/2 (796 eV), Co 2p3/2 (780 eV) and O 1s (530 eV). The fluoride species, with its stronger electron binding capacity than oxygen and very similar anion radius between the fluoride anion (F , 133 pm) and oxygen anion (O2 , 140 pm), existed as substitution doping in the p-type Co3O4 semiconductor.34 These fluoride species helped to improve the conductivity of the semiconductor, which was essential to enhance gas-sensing properties of semiconductor gas-sensing materials at a low temperature. In order to verify the effect of surface fluorine doping, control experiments by modifying C-Co3O4 with fluorine doping were conducted (see Experimental section, ESI†). The results demonstrated that fluoride-doped Co3O4 (denoted F-Co3O4) had higher response sensitivity than C-Co3O4 under the same test conditions (Fig. S10, ESI†). The optimum operating temperature of F-Co3O4 tended to be lower than C-Co3O4 (Fig. S11, ESI†). This indicated that the CO gassensing performance of the p-type Co3O4 semiconductor could be greatly improved by the surface fluorine doping method. In summary, we produced fluoride-doped porous Co3O4 nanowires from a fluoride anion assisted hydrothermal method and a subsequent controlled annealing route. In gas-sensing applications, the nanowires showed excellent CO gas-sensing performances compared to other Co3O4 nanostructured materials reported in the literature, including optimal sensitivity at a much lower operating temperature (Top r 100 1C) and excellent selectivity
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